A cosmetic reduction.
6,000 seated and 3,999 on the terraces - yeh, beats the 10,000 rule - but presumably with all those people in the ground, players, subs, managers, kit men, referees and assistants, physios, coaches, assistant managers, first aisers, stewards, hospitality staff, etc. would take the number in the stadium to well over 10,000, so wouldn't be allowable.